The present invention relates to a non-contact electrical power transmission system which includes a transformer with separable primary and secondary windings. The system transmits electric power from the primary side to the secondary side of the transformer in a condition in which a core wrapped with a primary winding and a core wrapped with a secondary winding remain out of contact with each other.
Heretofore, in many places, a non-contact electrical power transmission system has been commercially applied. Such systems configure a transformer in which the primary winding and the secondary winding are separable from each other with a core wrapped with a primary winding and a core wrapped with a secondary winding. The system transmits power from the primary side to the secondary side by electromagnetic induction with the primary core and the secondary core remaining out of contact with each other.
However, in most conventional non-contact electrical power transmission systems, a load connected between output terminals at the secondary side of a transformer has been specified. There has not been discovered a commercially applied example of a non-contact electrical power transmission system which is applicable to plural kinds of loads as a load connectable between output terminals, and also even to a kind of load whose current changes over a wide range.
Meanwhile, such non-contact electrical power transmission systems transmit an electric power from the primary side to the secondary side with an electrical insulator between the primary side of the above-mentioned transformer as electric power supply side and the secondary side having a load connected between output terminals. The degree of magnetic coupling of transformer is low, the magnetic flux interlinked to the secondary winding is fewer than that generated in the primary winding, and a leakage inductance develops due to leakage magnetic flux.
Although the frequency of a high-frequency AC voltage supplied to the primary winding of such transformers is generally in the audible-range frequency or above (about 20 kHz or more), the above-mentioned separable transformer is low in the degree of magnetic coupling and has a leakage inductance, whereby the induced voltage in the secondary winding is reduced and a voltage drop in induction reactance due to leakage inductance develops. As a result, the voltage supplied to a load may be smaller than a desired load voltage, or the current flowing to a load may be smaller than a desired load current. Explaining with a specific example, where various plural kinds of devices with a constant load voltage and variable load current are applicable to a load, the larger the load current of a load, the lower the voltage across the load becomes, whereby the performance inherent in devices cannot be exhibited.
Referring to FIG. 39(A), a prior-art rectifier circuit 7 is connected to the secondary side of a separable transformer T. A circuit for supplying a load current I to a load 10 consists of a variable resistance in series with a choke coil LCH. A high-frequency AC voltage is applied by an inverter 3 to the primary side of the transformer T. A secondary winding n2 of the transformer T includes a center tap 5e. One terminal of the load 10 is connected to the center tap 5e. The ends of the secondary winding n2 are connected to the anodes of diodes D2 and D3. The cathodes of the diodes D2 and D3 are connected together, and their junction is connected to the choke coil LCH. A capacitor C3 is connected in parallel with the load 10.
In the circuit shown in FIG. 39(A), a high-frequency square-wave AC voltage, having a maximum amplitude 70 volts and a frequency of about 97 kHz as shown in FIG. 39(B) is applied to a primary winding n1 of the transformer T. With the inductance value of the choke coil LCH taken as 100 xcexcH, the capacitance of the capacitor C3 connected parallel to the load 10 taken as 100 xcexcF, a gap g between a primary core 5c of the transformer T and a secondary core 5d taken as 2 mm, measuring a load voltage (output voltage)/load current characteristics and a load power/load current characteristics by changing variously the resistance value of the load 10 causes characteristics to be obtained as shown in FIG. 41. In FIG. 41, the axis of abscissa is for a load current 1, the axis of ordinate on the left side for a load voltage V0, and the axis of the ordinate on the right side for a load power P, the curve V indicating the load voltage and the curve P indicating the load power.
The transformer T has a configuration as shown in FIG. 40, in which the primary winding n1 is separately wrapped at two leg portions of the U-type primary core 5c, the secondary winding n2 is separately wrapped at two leg portions of the U-type secondary core 5d, and the center tap 5e is provided at the middle point of the secondary winding n2. Now, the inductance value when viewed from the primary. winding terminals A-Axe2x80x2 of the transformer T is 112 xcexcH, the inductance value when viewed from the secondary winding terminals B-Bxe2x80x2 is 42 xcexcH, and the mutual inductance value between the primary winding n1 and the secondary winding n2 is 91 xcexcH.
It will be understood from FIG. 41 that as the load current I increases, the load voltage V0 substantially decreases monotonously, while the load power P becomes smaller in increased values (becomes saturated) as the load current I becomes larger. In a non-contact electrical power transmission system for charging load 10, a matching capacitor is connected in parallel or series with the secondary winding n2 of the transformer T in order to offset an effect due to the leakage inductance of the transformer T, thereby increasing an effective power taken from the primary side to secondary side of the transformer T (improving a power factor by load matching). Providing such a matching capacitor causes a power transmission efficiency to be significantly improved for a certain load, thereby allowing the system to be miniaturized. Therefore, the matching capacitor is an important component in commercially applying a non-contact electrical power transmission system.
However, in a non-contact electrical power transmission system provided with the above-mentioned matching capacitor, a problem exits in that for a load whose load current I varies largely, the load voltage V0 lowers remarkably compared with a case where no matching capacitor is provided. For example, in a system which has substantially the same circuit configuration as shown in the above-mentioned FIG. 39(A) and in which the secondary winding n2 of the separable transformer T is connected in parallel with a matching capacitor C2 as shown in FIG. 42(A), supplying a high-frequency AC voltage of the square-wave shape having a maximum amplitude 70 volt and a frequency of about 97 kHz as shown in FIG. 42(B) to the primary winding n1 of the transformer T and changing variously the resistance value of the load 10 consisting of a variable resistance causes a load voltage/load current characteristics and a load power/load current characteristics as shown in FIG. 44 to be obtained. Now, in FIG. 44, the axis of abscissa is for the load current I, the axis of ordinate on the left side for the load voltage V0, and the axis of the ordinate on the right side for the load power P, the curve V indicating the load voltage and the curve P indicating the load power. Hereinafter, a value obtained with (varying range of load voltage V0)/(varying range of load current) is referred to as the voltage change rate.
It will be understood from FIG. 44 that the more the load current I increases, the more the voltage change rate of the load voltage V0 becomes large. It will be also understood that as load current I increases, the load power P exhibits a characteristics having a peak at a certain load current value. Furthermore, it will be understood that in a load current region for a very small load current I, the load voltage V0 becomes large. In the circuit shown in FIG. 42(A), an equivalent circuit converted to secondary side using a voltage induced in the secondary winding n2 of the transformer T is expressed as shown in FIG. 43. A portion between an end to which a diode D2 of the secondary winding n2 in FIG. 42(A) is connected and the center tap 5e is expressed equivalently with a high-frequency AC source I a and an inductance L03 in FIG. 43, while a portion between the other end of the secondary winding n2 and the center tap 5e is expressed with a high-frequency AC source 1b and an inductance L04 in FIG. 43.
In the non-contact electrical power transmission system having characteristics shown in FIG. 44, as a method of making constant (stabilizing) the load voltage for a plural kinds of loads identical in load voltage and different in power, that is, different in load current, there is devised a method such as that of providing a feedback control circuit which detects a load voltage at the secondary side of the transformer T, compares said detected voltage with a reference value to amplify an error, transmits in non-contact fashion the error-amplified signal to the primary side of the transformer T, and controls the amplitude, frequency and duty of the high-frequency AC voltage supplied to the primary winding n1 of the transformer T, or that of providing an independent stabilizing source circuit at the secondary side of the transformer T and connecting the circuit to the load.
However, providing such a feedback control circuit and a stabilizing source circuit causes the number of parts to increase and the cost to become higher. Now, the better the stability of a load voltage, the higher the effect obtained by providing these circuits becomes, thereby allowing a reduced number of additional parts to be expected. Hence, there has been expected an inexpensive non-contact electrical power transmission system having a relatively simple circuit which can make constant a load voltage (output voltage) in a wide load current without adding the feedback control circuit.
Furthermore, as a solution to a problem that as mentioned above, the larger the load current in a device, the lower the output terminal voltage becomes, thereby causing a performance inherent to the device not to be exhibited, for example, detecting the output terminal voltage and feeding back signals from the secondary side to the primary side to control allows the output terminal voltage to be stabilized.
However, even in such a manner, the relationship of the output terminal voltage and load power with the load current is such that as shown in FIG. 36, the load power P is substantially proportional to the load current I, and the output terminal voltage V0 is stabilized in a light load through full load region B, but it becomes rapidly larger in a no load through minute load region A. For non-contact charge or power transmission, in order to pick up a more amount of effective power even if a little on the secondary side of the separable/detachable transformer, a matching capacitor for load matching is often provided on the secondary winding side. It will be assumed that providing the matching capacitor causes the unique above-mentioned voltage rise different from the switching source due to normal contact coupling at the time of no-load/minute load. In order to control the output terminal voltage rise, dummy loads such as dummy resistances are connected parallel to the output terminal to generate a loss at all time to control, but in this method, a power loss at the dummy voltage becomes several watts or more, and the circuit size becomes large or the cost increases in order to control the efficiency decrease and temperature rise.
It is an object of the present invention to provide an inexpensive non-contact electrical power transmission system capable of making constant a load voltage in a wide load current range without making complex the circuit configuration.
It is another object of the present invention to provide a non-contact electrical power transmission system capable of controlling the output terminal voltage rise even at the time of no load/minute load, and capable of making lower a loss even a case of connecting the dummy load.
The non-contact electrical power transmission system according to the present invention comprises a transformer having a primary winding and a secondary winding separable and detachable therebetween, a capacitor connected parallel to the secondary winding of the above-mentioned transformer, and an output terminal provided on the above-mentioned secondary winding side and connected with a load, and supplies a high-frequency AC voltage to the above-mentioned primary winding to flow a high-frequency current to the above-mentioned primary winding, and generates an induced voltage on the secondary winding by the electromagnetic induction action, whereby a power is supplied to the load connected to the above-mentioned output terminal. This system is configured such that a voltage supplied to the above-mentioned load is substantially constant, a flowing current varies, and a power is supplied to a different load, and in the system, taking as a first condition a fact that at the time of maximum load, the time of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the time of the oscillating voltage of the above-mentioned capacitor reaching its maximum or minimum value, and as a second condition a fact that at the time of minimum load, the time of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the time of the oscillating voltage of the above-mentioned capacitor completing one cycle, the above-mentioned capacitor is set so that its capacitance satisfies simultaneously the above-mentioned first and second conditions, thereby making constant the load voltage in a minimum through maximum load current range. The above-mentioned configuration allows the load voltage to be made constant in a wide load current range with an inexpensive circuit configuration without providing the feedback control circuit, that is, without making complex the circuit. Therefore, a substantially-constant load voltage is supplied to a load whose load current largely varies or to plural kinds of loads whose load voltages are constant and load currents are different from each other.
This system may be configured such that in the system, instead of the above-mentioned second condition, taking as a second condition a fact that at the time of minimum load, the timing of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the timing of the oscillating voltage of the above-mentioned capacitor starting oscillation, and at the same time, the timing of the reversal of the voltage polarity in the above-mentioned primary winding substantially coincides with the timing of the oscillating voltage of the above-mentioned capacitor completing one cycle, the above-mentioned capacitor is set so that its capacitance satisfies simultaneously the above-mentioned first and second conditions. In this system, the circuit constant is set so as to satisfy a condition formula of 4xc2x7xcfx80xc2x7fxc2x7(L02_EC2)xc2xd=1 when the leakage inductance converted to the secondary side of the above-mentioned transformer is expressed as L02, the capacitance of the above-mentioned capacitor as C2, and the frequency of the above-mentioned high-frequency AC voltage as f. This allows the output voltage to be made constant for a load current equal to or less than the maximum value in the above-mentioned load current range. This system, in which a dummy load for flowing a current equal to or more than the above-mentioned minimum value even in the load current region smaller than the minimum value in the above-mentioned load current range is connected between the above-mentioned output terminals, can flow a current equal to or more than the above-mentioned minimum value even in the load current region smaller than the minimum value in the above-mentioned load current range. This system includes a drive circuit for supplying the above-mentioned high-frequency AC voltage to the above-mentioned primary winding, and in the above-mentioned drive circuit, the frequency of the above-mentioned high-frequency AC voltage changes automatically so that when the above-mentioned load current is within the above-mentioned load current range, the voltage supplied to a load is made constant. Hence, the load voltage is made constant in a wide load current range.
This system includes a drive circuit for supplying the above-mentioned high-frequency AC voltage to the above-mentioned primary winding, the above-mentioned drive circuit consisting of a resonance-type inverter. The above-mentioned drive circuit may be a partial resonance-type inverter having a resonating capacitor which is connected parallel to the above-mentioned primary winding and develops a resonance between the capacitor and the above-mentioned primary winding. This allows the load voltage to be made constant in a wide load current range while maintaining a soft switching. The above-mentioned drive circuit may be the one which includes a voltage resonance circuit by the above-mentioned primary winding and by a resonating capacitor connected parallel to the above-mentioned primary winding, and the voltage waveform of the above-mentioned high-frequency AC voltage exhibits a sinusoidal waveform. The above-mentioned drive circuit should be such that in a period when the on-time of a switching element switched in said drive circuit is constant, and a partial resonance develops, and at the same time, in at least one period of either the rising period or the falling period of the voltage waveform of the above-mentioned high-frequency AC voltage, at least one of either the time of said period or the voltage waveform of said period varies corresponding to the load current. This allows the load voltage to be made constant in a wide load current range while maintaining a soft switching.
The above-mentioned inverter can employ a half-bridge type one or a push-pull type one. This causes the utilizing efficiency of the transformer core to be improved. The above-mentioned inverter may be a self-excited one which includes a feedback winding and an auxiliary winding each magnetically coupled to the primary winding of the above-mentioned transformer, a voltage-drive type switching element given an input voltage the control end through the feedback winding, and a charging/discharging circuit connected between both ends of the auxiliary winding for controlling the above-mentioned input voltage, and when a charge voltage due to an induced voltage of the auxiliary winding reaches a specified value, lowers the above-mentioned input voltage to turn off the above-mentioned switching element. This causes the rising period and falling period of, and the waveform of the voltage to vary utilizing a change in the resonance state of the voltage of the primary winding developed in the off-time of the switching element corresponding to the load current, thereby allowing the load voltage to be made constant in a wide load current range. Even in a load current region smaller than the minimum value in the above-mentioned load current range, a resistance for flowing a current equal to or more than the above-mentioned minimum value is sufficient to be connected between output terminals connected with a load. This allows the output voltage to be automatically made constant in all load current ranges.
Furthermore, the non-contact electrical power transmission system according to the present invention comprises an inverter circuit including a transformer having a structure of the primary winding and the secondary winding whose voltage is induced by the primary winding being separable and detachable therebetween, a first capacitor connected to the above-mentioned secondary winding side for being matched with a load, a rectifier circuit for rectifying a voltage induced in the above-mentioned secondary winding, a current-smoothing reactor for smoothing an output current of the above-mentioned rectifier circuit, and an output terminal supplied with a smoothed output by the above-mentioned reactor and connected with the load, and in the system, an inductance value of the above-mentioned reactor is selected so that when the magnitude of the above-mentioned load is made changed, the load current value at the time when the output voltage of when the magnitude of the above-mentioned rectifier circuit varies from a discontinuous condition to a continuous condition is made smaller, whereby the rise of the above-mentioned output terminal voltage at the time when the above-mentioned load is no-load or minute load is controlled.
A dummy load capable of flowing at all time a load current value at the time when the output current of the above-mentioned rectifier circuit varies from a discontinuous condition to a continuous condition may be connected to the output terminal. A second capacitor is connected parallel to the above-mentioned current-smoothing reactor, and an electrostatic capacitance of the above-mentioned second capacitor is set so that the AC voltage component of the voltage at the input side of the above-mentioned current-smoothing reactor exhibits a sinusoidal waveform. This allows the rise of the output terminal voltage to be controlled even at the time of no-load/minute load and a loss to be made lower even a case of connecting a dummy load.
In this system, the electrostatic capacitance of the above-mentioned second capacitor is set so that when the load is increase gradually from no-load, the amplitude of the AC voltage component of the voltage at the input side of the above-mentioned current-smoothing reactor becomes equal to that of the output terminal voltage. The electrostatic capacitance of the above-mentioned second capacitor may be set so that when the load is increased gradually from no-load, the load current value at the time when the zero period in which the output current of the above-mentioned rectifier circuit is zero dissipates becomes smallest. A resonance frequency determined by the inductance value of the above-mentioned current-smoothing reactor and by the electrostatic capacitance of the second capacitor is equal to two times the frequency of the voltage applied to the primary winding. A resonance frequency determined by the electrostatic capacitance of the above-mentioned first capacitor and by the leakage inductance value converted to the secondary side the separable and detachable transformer may be equal to two times the frequency of the voltage applied to the primary winding. The above-mentioned secondary winding includes a center tap, and the rectifier circuit consists of two diodes, and connects one end of the above-mentioned each diode in series and in opposite direction to each other to both output ends (not to the center tap) of the above-mentioned secondary winding, thereby configuring a full-wave rectifier circuit interconnecting the other ends of the above-mentioned each diode. When the magnitude of the load is made changed by making larger the inductance value of the above-mentioned current-smoothing reactor, the load current value at the time when the output current of the above-mentioned rectifier circuit varies from a discontinuous condition to a continuous condition is made smaller, thereby controlling the rise of the output terminal voltage at the time of the load being no-load or minute load.